Spark plugs are used to initiate combustion in internal combustion engines. Spark plugs typically ignite a gas such as, for instance, an air/fuel mixture in a combustion chamber or combustion space by producing a spark over a spark gap that is defined by two or more spark plug electrodes. The igniting of the gas by means of the spark produces a combustion reaction, which in turn drives a piston inside an engine cylinder during a power stroke of the combustion process.
In modern spark plugs, seals are used at different locations in the spark plug so that the spark plug installed in the cylinder head or engine or spark plug hole is gastight with respect to the gases located in the combustion chamber. In addition to an outer seal for sealing the spark plug housing with respect to the cylinder head or engine, there is at least one inside seal, also called inner sealing washer or inner gasket, which seals the transition between the metal shell, in which the thread is accommodated, and the insulator that surrounds the center electrode.
Because of the specific requirements on an inside seal, such as, for example, temperature resistance, thermal conductivity, and plastic deformability, until now metal seals have been used, which are made, in particular, of aluminum alloys, copper alloys, or steel alloys.
It has become apparent, however, that in the case of metallic inside seals, undesirable voltage flashovers between the metal shell and the center electrode located in the insulator sometimes occur in the region of the inside seal. Such undesirable voltage flashovers lead to misfires in the combustion chamber, since the electrical energy is dissipated in the interior of the spark plug. The spark plug is damaged. This has the consequence that no ignition spark is produced in the spark gap (between the center electrode and ground electrode). The energy for ignition of the fuel/air mixture is not made available at the spark gap.
Such undesirable flashovers occur in prior art spark plugs because the metallic inside seal is at the same electric potential as the metal shell, and consequently reduces the effective distance between the metal shell and the center electrode, which are at different levels of electric potential. Also, for manufacturing reasons the metallic inside seals can have edges at which local increases in the electric field can occur. The field increase can extend into the insulator, and raises its electrical demands, reducing the electrical strength of the spark plug.
While the dielectric strength of the insulator can indeed be influenced by the choice of material, which is to say the choice of the applicable ceramic, there are nonetheless material related limits in this regard. Typical dielectric strengths of insulators used in spark plugs they are generally made of aluminum oxide are on the order of 19 kV/mm.
In order to increase the dielectric strength of a spark plug, it would thus be desirable to produce the sealing ring of the inside seal from a material with high dielectric strength.
From WO 2015/090,723 A1, a spark plug is known with a sealing ring for the inside seal that is made entirely of nonconductive material. Two inside seals are used in this design: a first inside seal in the region of the end on the combustion chamber side of the transition between the metal shell and the insulator, in particular in the region of the neck of the insulator nose, as well as a second seal in the region of the end of the metal shell facing away from the combustion chamber.
The sealing rings in question are made of a polymer or an oxide ceramic material, such as strontium titanate, for example.
While oxide ceramics have the disadvantage that they are not plastically deformable and a seal with them can therefore be achieved only with difficulty, polymers have the disadvantage that they frequently are not sufficiently temperature resistant. Even though polymers also exist that are temperature resistant up to 300° C. and above, they usually do not have adequate thermal conductivity.
Otherwise, it is not possible to learn from this publication which polymer is supposed to be suitable for the sealing ring in question.
It is therefore desirable to provide a spark plug with an increased dielectric strength.
It is also desirable to disclose a spark plug with a long service life.